I was reminiscing on Halo 3's nostalgia when my mind came to the Gravemind's inquiry to the Master Chief in the level Cortana, saying "Child of my enemy, why have you come? I offer no forgiveness, a father's sins, passed to his son." and it struck me in the scope of the relationship of the Precursors, Forerunners, and Humanity.

To recap, the Mantle of Responsibility was originally intended for Humanity eons ago, yet the Forerunners rebelled against the Precursors in order to secure it for themselves. Humanity then weren't aware that they should have been the original keepers of the Mantle, so they went on to make a stand against the Flood and then the Forerunners, resulting in the Human-Forerunner War. After exhausting themselves, the Forerunners then had to wage against the Flood. In the end, the Forerunners and the Librarian bestowed Humanity with the Mantle of Responsibility in time, in which the ur-Didact didn't accept.

So when the Gravemind confronted the Master Chief in the level Cortana, he was referring to this fact. And this revelation was confirmed with Guilty Spark's inability to accept Installation 4's sacrifice at the Lesser Ark by ranting "...you are a child of my makers, inheritors of all they left behind. You are Forerunner, but this ring... is mine!”

Between these two quotes, the Gravemind recognized that Humanity posed to be the greatest player in the Milky Way after the Human-Covenant war came to a close. As well as the ur-Didact's epilogue in Halo 4.

"In this hour of victory, we taste only defeat. I ask, why? We are Forerunners, guardians of all that exists. The roots of the galaxy have grown deep under our careful tending. Where there is life, the wisdom of our countless generations has saturated the soil. Our strength is a luminous sun, towards which all intelligence blossoms... And the impervious shelter beneath which it has prospered. I stand before you, accused of the sin of ensuring Forerunner ascendancy. Of attempting to save us from this fate where we are forced to... recede. Humanity stands as the greatest threat in the galaxy. Refusing to eradicate them is a fool's gambit. We squander eons in the darkness, while they seize our triumphs for their own. The Mantle of responsibility for all things belongs to Forerunners alone. Think of my acts as you will. But do not doubt the reality: the Reclamation... has already begun. And we are hopeless to stop it."